It is essential for the operation of a practical wireless communication network to support the communication of control information between a base station and the user terminals that it supports. By “control information” is meant any information related to the operation of the network and its components, as distinct from content, i.e., from the information which is to be communicated between users.
Control information to be communicated on the forward link, i.e. from the base station to the user terminal, may include, for example, acknowledgement bits and power-control bits. An acknowledgement bit indicates to the user terminal whether a message sent in a particular time window on the reverse link, i.e., from the user terminal to the base station, was successfully decoded at the base station. As is well known, acknowledgement bits may be positive, i.e., indicative of successful decoding, or negative, i.e., indicative of an unsuccessful decoding attempt. A power-control bit indicates to the user terminal that transmission power on the reverse link should be adjusted up or down. Those skilled in the art will appreciate that numerous other types of control information may also be communicated on the forward link.
It is well known that no communication channel has unlimited capacity. Instead, the consumption of bandwidth and power resources by control signaling will often limit the extent to which those resources are available for transmitting content. As a consequence, there is an economic incentive to minimize signaling overhead, i.e., the extent to which control signaling takes up bandwidth and power resources.
For example, OFDMA systems such as those that conform to the standard IEEE 802.1 6e assign frequency resources for transmitting control information on the forward link. Each signaling bit may be sent on a particular subcarrier or set of subcarriers reserved for a particular user. Alternatively, signaling bits may be pooled and sent on a common set of reserved subcarriers. In this second scenario, the signaling bit or bits for a particular user are appended to an identification word for that user, so that at the receiving end, each user can identify its own destined control information.
When each user has one or more dedicated subcarriers, there will typically be sufficient dedicated bandwidth to process that user's control messages at the peak rate. However, control information is not necessarily transmitted to a given user at a constant rate. Instead, it may be sent sporadically. For example, in some systems acknowledgment bits will be sent only in response to successful decoding attempts, as explained above. As a consequence, signaling bandwidth is used inefficiently.
On the other hand, the use of pooled bandwidth according to the second of the scenarios described above is inefficient because it requires user identification bits to be sent together with the control information. Since there are typically many more users than there may be control bits per user, the user identification bits may constitute a substantial amount of overhead.
For the systems described above, and others, there remains a need for improving the efficiency of forward-link control signaling.